As drugs having smooth muscle relaxation activities, drugs acting on a contraction system and those acting on a relaxation system are known. As the drugs acting on a contraction system, there are .beta.-blockers, .alpha..sub.1 -blockers, calcium antagonists and the like. As the drugs acting on a relaxation system, there are nitro compounds, and the like.
A new type of drug called a potassium channel opener has recently been noted. The drug opens (activates) potassium channels to exhibit smooth muscle relaxation activities.
As an effect to be expected for a potassium channel opener, for example, when hypotensive activity is considered, the drug can exhibit vasodilation activity without any influence on calcium channels and any function based on calcium channels of other organs (e.g., heart) is not impaired. The potassium channel opener can therefore be a drug which has few side effects of inhibitory activity on heart function and has potent hypotensive activity.
For example, chroman-3-ol derivatives which have potassium channel opening (activating) activity and exhibit hypotensive activity on spontaneously hypertensive rats are disclosed in JP-A 58-67683, J. Med. Chem., 29, 2194-2201 (1986) and Br. J. Pharmac. 88, 103-111 (1986). Further, other compounds having potassium channel opening activities are also disclosed in JP-A 57-130979, EP-A 298452 and EP-A 371312.